Plasma generators of this type which are known at the present time generally have a cathode which is hot, in other words heated electrically, and which terminates in a suitably calibrated nozzle through which the gas supplied through the cavity of the cathode exits. In the terminal region of the cathode, near the nozzle, there is disposed an insert of porous material, for example tungsten, impregnated with materials with a low work function, for example oxides of barium or of calcium.
The hot cathodes of these generators are critical components of the devices, since they are extremely sensitive to the presence of oxygen, moisture and other contaminants present in the gas supplied to their interior or in the environment. For this reason, it is necessary to provide an extremely sophisticated gas supply line, which in most cases comprises expensive devices for eliminating impurities, which devices have to be periodically checked and replaced. Furthermore, the hot cathode requires the presence of a power supply unit, which forms an additional element of the system, which is susceptible to failure, and in any case increases the weight and power consumption of the system. This is a disadvantage particularly for applications in space, and typical of these plasma generators. Another disadvantage of the hot cathode plasma generators is related to the presence of high temperatures in the cathode and in the surrounding regions, which dictates the use of compatible materials, components and processes and requires precise technical design.